1. Field of the Invention
The present invention relates to a buffer controller, and more particularly to a buffer controller applicable, for example, to a session border controller (S/BC) provided on a telecommunications network border.
2. Description of the Background Art
In an Internet protocol (IP) interconnection environment, such as next generation network (NGN) using an IP interconnection, not only pure data but also media data such as audio and/or video data may be transmitted between different personal computers (PCs) or terminals on specific service provider networks. Service provider networks, which are equipped with codecs different from each other, require codec conversion for converting data transmitted from a sender service provider network to data appropriate for another service provider network to which the data are destined.
Such a codec conversion is disclosed by U.S. patent application publication No. 2009/0268755 A1 to Inoishi. In the codec conversion taught by Inoishi, when receiving packets such as real-time transport protocol (RTP) packets, a codec conversion device places the packets into a network buffer, i.e. the NW buffer shown in its FIG. 1, so that the NW buffer temporarily stores the packets in sequence. The packets thus stored are then successively read out in sequence from the NW buffer and distributed over resources by a resource allocator. Codec conversion will then be performed in respective decoder/encoders to transmit the packets.
Inoishi also teaches that it takes account of the fact that the most of all backend terminals on an end-to-end communication execute processes for correcting the order of incoming packets and compensating for lost packets, and such terminals are possibly be burden with those processes so as to design a codec conversion device not executing the processes to thereby reduce a delay otherwise arising from the codec conversion.
However, if the packet order correction and packet loss compensation are not performed by a codec conversion device, although done by end-to-end communication terminals, sound quality would deteriorate in the case of VoIP (Voice over Internet Protocol) telephony, as will be described with reference to FIG. 12.
FIG. 12 schematically shows an example of a state where sound quality deteriorates in a codec conversion device when packets are out of sequence. In the figure, the codec conversion device 91 receives packets from a transmitter terminal 92 and performs codec conversion from AMR (Adaptive Multi-Rate) to ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G.711 format to transmit resultant packets to a destination terminal 93.
The sender terminal 92 partitions an original sound signal, for example, at intervals of 20 ms into original sound segments (1), (2), (3) and (4), and successively encodes them by AMR to form encoder output segments respectively referred to as AMRs (1), (2), (3) and (4). They are sent out successively from the terminal 92 to the codec conversion device 91 in the form of packets. If the packets with the sequence numbers “2” and “3” are out of sequence as indicated by arrows 200 and 201, that is, both packets are permuted during transmission, the codec conversion device 91 receives the AMR (3) earlier than the AMR (2). Therefore, the decoding order in the codec conversion device 91 is AMRs (1), (3), (2) and (4), and the codec conversion device 91 decodes the packets in the latter order, rather than the original order indicated by AMRs (1), (2), (3) AMR (4).
In this way, the packets are out of sequence, so that linear sounds (3) and (2) output from the decoder become different from the original sounds. That raises a problem of deterioration in sound quality because an audio codec such as AMR is adapted to additionally use information obtained from the past audio data for encoding/decoding. Therefore, if the receiver terminal 93 tries to correct the packet order to the original order, it would be impossible to completely restore the sound quality.
Note that in order to reduce the sound quality deterioration, a jitter buffer is disclosed, for example, by Japanesepatentlaid-openpublication No. 2005-64873. It would, however, take an extra period of time from when the jitter buffer receives a packet until it develops the packet in addition to the codec conversion, thereby causing delay.